JP2008-175281A describes this type of hydraulic control apparatus.
This conventional hydraulic control apparatus includes a valve main body into which a plurality of spools are incorporated, a plurality of caps provided in accordance with the plurality of spools, pilot chambers provided in the respective caps, and an oil passage plate interposed between the caps and the valve main body. A plurality of plates constituting the oil passage plate are integrated using bolts or the like. The bolts are provided on an outer side of the pilot chambers to avoid interference with the pilot chambers. A pilot pressure is led to the pilot chambers, and the spools are switched by an action of the pilot pressure.
A maximum pressure introduction passage is formed in the oil passage plate in an orthogonal direction to axes of the respective spools. The maximum pressure introduction passage is formed in an offset position from the pilot chambers, and therefore the pilot chambers and the maximum pressure introduction passage maintain a non-interfering positional relationship.
Selection valves for selecting a maximum pressure are provided in connection passages connecting the maximum pressure introduction passage to the respective pilot chambers. The pilot pressure in the pilot chamber acts on one side of the selection valve, and a pressure in the maximum pressure introduction passage acts on another side.